1. Field Of The Invention
The present invention relates generally to the field of submergible pumping systems for producing fluids from wells, particularly petroleum production wells. More particularly, the invention relates to a novel technique for driving and controlling a submergible pumping system at a range of speeds, thereby permitting flow rates from the pumping system to be varied. The system is particularly well suited for driving pumping systems including progressive cavity pumps and similar devices having relatively high starting and low speed torque requirements.
2. Description Of The Related Art
A variety of systems are known for producing viscous fluids from petroleum production wells and the like. Where the well formations provide sufficient pressure to raise wellbore fluids to the earth's surface without the aid of pumps, the well may be exploited directly, such as by appropriately equipping the wellhead with valving, transfer conduits, and so forth. However, in many production wells pressures are insufficient to raise the production fluids to an above-ground collection point. Consequently, pumping systems are often employed within the well for drawing the fluids from the well formations, separating the fluids in situ, if required, and raising the production fluids to the earth's surface for subsequent collection and processing.
In one known class of pumping systems of this type, a submergible pumping unit is immersed in the wellbore fluids and driven to force fluids through a production conduit to the earth's surface. Such systems typically include a submergible electric motor, a production pump, and related equipment for protecting the motor and sealing portions of the wellbore where necessary. Such systems may also include fluid or gas separators, injection pumps, and other ancillary components.
In submergible pumping systems of the type described above, centrifugal pumps are commonly employed for producing the wellbore fluids. While in many applications such pumps provide sufficient lift and adequate efficiencies, a number of applications exist where their performance is less than satisfactory. In particular, in wells producing heavy or viscous fluids, centrifugal pumps may not develop sufficient pressure head to adequately displace the fluids through the production conduit. Moreover, depending upon well production rates, it may be desirable to vary the flow rate of fluid displaced by the pump by adjusting the speed of the production pump. For example, depending upon availability of collection vessels, flow rates from the well formations and so forth, the well operator may desire to reduce production rates from the well during certain periods, and to increase production substantially during other periods. However, because centrifugal pumps are typically inefficient at lower speeds, their use in submergible pumping systems may limit the range of production rates available to the well operator, particularly at low speeds.
Alternative solutions to the use of centrifugal pumps have been proposed and are currently in use. In one known approach, a positive displacement pump, such as a progressive cavity pump is employed in the place of a centrifugal pump. Such pumps offer a significant advantage over centrifugal pumps in that they displace viscous fluids very effectively over a wide range of speeds, including at low speeds. However, unlike centrifugal pumps, which have very low starting torques that can be provided directly by a submergible electric motor, progressive cavity pumps require significantly higher torques within a low speed range. This high torque requirement poses problems both during starting of the pumping system and during periods when production rates are reduced to relatively low levels.
To provide sufficient starting and low speed torque for progressive cavity pumps, known submergible pumping systems for wells typically employ a gear reducer for increasing output torque of a submergible electric motor coupled to the pump. The gear reducer is specially designed to fit within the space constraints of the wellbore, and is positioned in an intermediate module between the electric motor and the progressive cavity pump. The electric motor is typically a polyphase induction motor, which may be driven by various control circuits capable of varying its running speed. Such circuits include conventional inverter drives and the like.
During operation, the gear reducer acts as a torque multiplier (and concomitantly as a speed reducer), permitting the progressive cavity pump to be started by the electric motor and to be driven at a reduced speed. However, gear reducers are typically employed with a fixed operating speed which is lower than may be desired during certain phases of operation. Moreover, even where a variable speed motor drive is used, such gear reducers limit the range of speeds at which the pump can be driven, typically making higher production rates unavailable. Consequently, while pumping systems employing gear reducer-driven progressive cavity pumps may offer sufficient torque for starting the pump and for pumping at lower speeds, they do not offer the well operator the flexibility to pump fluids from the well at both lower and higher flow rates.
There is a need, therefore, for an improved technique for pumping fluids from wells via submergible pumping systems. In particular, there is a need for a system capable of effectively controlling a progressive cavity pump over a wider range of speeds than can be attained by heretofore known control systems. There is also a particular need for a control technique for such pumps which reduces electrical power losses during operation, while providing sufficient power to satisfy the starting and low speed torque requirements of the pumps.